Film projection systems used in commercial applications, such as movie theaters or theme parks, typically include a lamp house and a projector head. The projector head houses a projection lens assembly for projecting a focussed image upon the screen. The lens assembly is positioned close to a film gate which positions the film precisely in relation to the optical axis and focal point of the lens assembly as the film is advanced frame by frame past the gate. The lamp house houses the light source, typically an electric light, positioned near the focal point of a reflective collector. Nowadays, the collector is usually in the shape of the lower part of an ellipsoid or a sphere.
In a commercial projection system, alignment is critical for three optical components: the collector, the film gate, and the lens mount. Each of these components has an optical axis. When the optical axis of each of these components is collinear, the system optical axis is established. It is necessary that the optical axis of the collector, the optical axis of the film gate, and the optical axis of the lens mount, all lie along the system optical axis for proper projection system alignment.
The optical axis of each component can be misaligned with respect to the system optical axis in three ways: it can be off center, it can be tilted, or it can be off center and tilted.
In many modern projectors, a folded projection system is employed. In the folded projection system, a flat mirror is used to fold the system optical axis. Typically the flat mirror is inclined at 45.degree. and is called the folding mirror. The folding mirror is positioned centrally over the light source and collector. Through reflection, the folding mirror bends the system optical axis 90.degree..
In commercial projection systems, it is very important that the light from the light source and collector should be directed evenly and centrally over the opening through the film gate. If the collector is misaligned off center or tilted relative to the optical axis of the film gate or relative to the optical axis of the projection lens, the illumination across the film gate will not be uniform and the quality of the projected image viewed by the audience will be visibly unsatisfactory. Misalignment can be, for example, a particular problem in multiscreen projection where different projectors are projecting edge-to-edge images on adjacent screens, since a noticeable difference in light intensity across the edges of two adjacent screens, caused by improper alignment of the light source in one of the projecting systems, will undermine the impression of a seamless panorama that the audience is intended to experience. Accordingly, it is important to be able to adjust a film projection system to overcome any optical misalignment that may exist.
An early technique for correcting misalignment simply used a taut string stretched along the optical path to align the components. This technique was used in projection systems that were not folded. The lamp constituting the light source would be temporarily removed from its socket and a string would be attached to the middle of a plate placed in the center of the collector's lamp hole directly through a centered aperture in the film gate to the middle of a plate centered in the mount for the lens assembly. The string was drawn tight so that the various parts could be aligned directly with reference to the datum provided by the taut string. With the advent of folded projection systems, however, the string method was no longer functional because the string cannot represent the system optical axis, since the optical axis is reflected through a 90.degree. change of direction by the folding mirror.
Alternative techniques were developed for folded projections systems based upon the use of a laser beam to achieve the desired alignment. One such prior art technique, shown in FIG. 2, involved the use of an a.c. powered helium neon laser that was aligned with the lens mount assembly, with the lens assembly removed. (This system was developed by the assignee of the present application, The Walt Disney Company.) The laser directed a laser beam along the optical axis of the lens mount, through the film gate, to the folding mirror which reflected the beam to the collector. A mechanical target/mirror assembly was installed in the lamp hole at the base of the collector. The target had a mirror with the center of the collector indicated by a reflective cross engraved upon the mirror. The target was installed to place the center of the cross at the center of the optical axis of the collector. However, there was no way to determine that the target was not tilted with respect to the optical axis of the collector. By adjusting the position of the collector within the lamp house, in a plane perpendicular to the incident laser beam, the cross on the target could be moved beneath the spot of light produced by the incident laser beam. This centered the collector relative to the optical axis of the lens mount. An attempt to correct tilt of the collector was made by tilting the collector and target assembly until the reflected beam from the target was coincident with the incident beam. This method of removing collector tilt was inaccurate since the target/mirror assembly used the lamp hole as a datum that is perpendicular to the collector's optical axis. While a collector's lamp hole is centered well enough for alignment purposes, the plane that the lamp hole lies in is not perpendicular to the optical axis. The tilt angle of the plane that the lamp hole lies in is large enough to cause significant misalignment. This hole is not precision machined or formed. It is ground to remove edges after the collector is formed during manufacturing. This angle varies significantly even amongst collectors of the same model.
While this approach helped in achieving better alignment, it failed to solve one significant problem. Although the optical axis of the collector was now properly centered, it might be tilted relative to the optical axis of the lens mount, leaving significant misalignment uncorrected.
The next refinement in the prior art, dependent upon the use of a laser beam for alignment, is shown in FIG. 3. In this technique, the upper peripheral edge of the collector must be made with a lip precisely formed to lie in a plane perpendicular to the optical axis of the collector. An alignment tool is placed on this lip, having a cross bar of the correct diameter to fit onto it. The cross bar is made of transparent material and has a pin hole at its center. Underneath and surrounding the pin hole is a transparent cylinder supporting a target mirror perpendicular to the optical axis of the collector. To use this system, an a.c. powered helium neon laser projects a laser beam along the first optical path to the mirror which reflects the beam downwardly through the pin hole onto the target mirror which would reflect the beam back up towards the pin hole. If the collector and the mirror are tilted relative to each other, the beam reflected from the target mirror will strike the plastic surrounding the pin hole, providing a visual indication of misalignment. The tilt of the collector can then be adjusted until the laser beam reflected by the target is aligned with the pin hole.
While the system of FIG. 3 provides some improvement, it still has very significant limitations. A particularly severe limitation is that it depends upon the manufacture of a specially configured lip on the upper peripheral surface of the collector, precisely configured to lie in a plane perpendicular to the optical axis of the collector. Such lips are not found on the great bulk of collectors in film projection systems already installed around the country nor are they present on most collectors currently manufactured. While a few collectors may have their upper rim in a plane precisely perpendicular to the optical axis of the collector, many others do not. Without knowing, for each collector, whether this is the case, a method of adjustment that depends upon this relationship being present cannot be relied upon. Another severe limitation is that, so far as is known, the adjusting tool of the prior art was made in only one diameter size. Since a typical operator of multiple film projection systems will possess collectors having a variety of different diameters in his inventory of systems, each different diameter collector would require a separately constructed aligning tool of different diameter.
Thus, at the present time, need exists for a method and apparatus for optically aligning film projection systems which is of widespread application to collectors without limitation whether the upper edges of the collectors have been cut in a plane that is accurately perpendicular to the optical axis of the collector and without limitation to a unique collector diameter.